Aluminum is lighter than steel but not as strong. So in order to make a frame with aluminum, one has to use larger diameter tubes. Large diameter tubes are much stiffer than narrow diameter tubes. This is why the early aluminum bikes had a reputation for having harsh rides. They did! Steel was seen as more flexible and more compliant because it was.
I understand that engineers now know how to design around the limits of the materials. So a good bike frame can be made out of aluminum, steel, titanium or carbon fiber. Right now, all the buzz gives carbon fiber the clear edge.